Pas de nombre d’or pour la biodiversité

11.09.2013 | par Vincent Devictor | Non classé

Insaisissable diversité naturelle. Il n’y a pas de diversité dans la nature. Il y a « des » diversités. Pour percevoir ce problème, prenons l’exemple ci-dessous tout simple. A gauche, il y a trois espèces. De ce point de vue, l’assemblage d’espèces est plus diversifié à gauche.


La diversité est-elle plus élevée à gauche ou à droite?

Cela dit, parmi ces espèces, deux ne sont représentées que par un seul individu. La diversité individuelle (« intra spécifique») est donc plus forte à droite et non à gauche. De plus, en faisant disparaître seulement deux individus à gauche (la libellule et la coccinelle) il ne reste plus qu’une seule espèce ! A droite en revanche, la diversité des espèces est plus « résistante » à un changement du nombre d’individus. Une autre réponse serait obtenue en considérant la diversité des couleurs, des tailles, ou encore la diversité génétique des assemblages de gauche ou de droite.

Cet exemple tout simple expose un problème qui hante les sciences de l’écologie et les problématiques de conservation de la nature : Comment mesurer « la » diversité ?

Il a fallut attendre beaucoup de débats et de recherches pour conclure qu’il n’y a pas de réponses simples à cette question car la réponse dépend des objectifs ! Que veut-on mesurer et pourquoi ? Par exemple, en France, la diversité des espèces d'oiseaux est plus forte dans les aires protégées. Bonne nouvelle. Mais la diversité des traits fonctionnels de ces mêmes oiseaux (comme la variabilité dans les régimes alimentaires, les modes de reproduction) est plus forte en dehors des zones protégées. Il ne suffit pas de protéger beaucoup d'espèces pour protéger toute la diversité ! Voila un dilemme clef de l'écologie aujourd'hui. Il faut pouvoir simplifier cette diversité sans néanmoins oublier sa complexité.

A l'échelle globale, il a fallut notamment se mettre d’accord sur la manière de mesurer « l’état de la nature ». Actuellement, un groupe d’experts sur « l’état de la planète » veille à établir un tableau de bord de ces mesures, multiples et complémentaires. Plus de 20 indicateurs permettent aujourd’hui de suivre l’état de la biodiversité sous toutes ses coutures.

Plus qu’un simple exercice de math ou de logique appliqué au vivant, cette recherche sur les indicateurs traduit notre façon d’envisager le devenir des espèces et des milieux naturels. Mais il ne s'agit pas d'attendre de tout transformer en indicateurs pour déployer des mesures de protections efficaces !

116 commentaires pour “Pas de nombre d’or pour la biodiversité”

  1. kamenova Répondre | Permalink

    bonjour vincent,
    je vois que tu t'amuses bien! 🙂 je ne savais pas que pour la science avait un blog sur l'écologie.
    et même que j'ai appris récemment que contrairement à ce que l'on puisse penser les patterns de la diversité spécifique et la diversité génétique chez de nombreux groupes taxinomiques ne corrèlent absolument pas. ça pose de vraies questions en termes de conservation, et notamment en termes de conception d'aires protégées!
    à bientôt.

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